Enoxaparin, a low molecular weight heparin, inhibits platelet-dependent prothrombinase assembly and activity by factor-Xa neutralization

Background: The available evidence suggests strongly that intravascular thr ombosis is mediated predominantly by tissue-factor and its activation of fa ctor X, which in the presence of factor Va, calcium, and phospholipid (prot hrombinase complex) effectively converts prothrombin to thrombin. In vitro experiments have shown that low molecular weight heparins (LMWHs) have grea ter anti-Xa activity than unfractionated heparin; however, it remains uncle ar as to whether their antithrombotic effects in vivo are determined by a s imilar mechanism. We determined the ability of plasma obtained from patient s with either unstable angina or non-ST segment elevation myocardial infarc tion (MI) receiving the LMWH enoxaparin (anti Xa:IIa ratio 3:1) to inhibit tissue factor-mediated thrombin generation and to inactivate platelet proth rombinase. Methods: Platelet rich plasma was prepared by suspending washed donor plate lets in the plasma of 7 patients participating in the TIMI 11A study. Sampl es were obtained before, 1 hour after a 30-mg IV bolus of enoxaparin and 6 hours after the third subcutaneous injection (1.0-1.25 mg/kg given subcutan eously every 12 hrs). Tissue factor (0.1 ng/ml) and 10 mM CaCl2 were added to initiate extrinsic coagulation. At timed intervals prothrombin activatio n fragment 1.2 (F1.2) levels (thrombin generation) were measured using an E LISA technique. Inactivation of reformed platelet prothrombinase by samples obtained at the same time points was also determined. Results: Patient plasma obtained 1 hr after treatment initiation and 6 hour s after the third subcutaneous injection inhibited tissue factor mediated p rothrombinase assembly by 31% and 11%, respectively and platelet prothrombi nase activity by 27% and 22%, respectively. Conclusion: We conclude that enoxaparin in plasma concentrations achieved r outinely in clinical practice is able to: (1) inhibit tissue factor mediate d extrinsic coagulation by preventing platelet surface prothrombinase assem bly, and (2) inactivate platelet prothrombinase activity and resulting thro mbin generation. These observations suggest that a LMWH's anti-Xa activity (and anti-Xa:IIa profile) is important in determining its overall antithrom botic potential. Clinical trials comparing agents with differing anti-Xa:II a properties will be required, however, to provide proof of concept.